Ringing significantly reduced after decreasing the length of the transmission line

[henry kissinger]

I have the simulation setup as below.

A source of data generator with source impedance 35 Ohm. DataRate 5 GHz with rise and fall time specified as below.
The characteristic impedance of the line is 60 in this setup which matches the 60 Ohm load impedance.

The result voltage at node "V" is shown below, even though the line impedance is matched to the load impedance, there is still ringing.

However if I decrease the length of the line, the ringing is significantly reduced. (the line impedance is not changed, or little if any if I change the length)
Why is this happening? The shorter the line, the less it rings? considering the load and the line are impedance matched already.

 

[barry]

Ringing is due to reflections AND propagation delay. Changing the length changes the propagation delay

 

[FvM]

Wrong assumption. As mentioned in your previous similar topic thread, reflections are observed because the load isn't matched, respectively line impedance not corresponding to the expected value over the signal frequency range.

As for the length effect, with 0.5 cm length, propagation delay is similar to signal rise time. Reflections are hidden in the edge then.

 

[barry]

Wrong assumption. As mentioned in your previous similar topic thread, reflections are observed because the load isn't matched, respectively line impedance not corresponding to the expected value over the signal frequency range.

As for the length effect, with 0.5 cm length, propagation delay is similar to signal rise time. Reflections are hidden in the edge then.

Even with a matched impedance there is still a reflection. It is absorbed at the source.

 

[FvM]

You should explain why.

 

[danadakk]

There is no relfection under these conditions :

Regards, Dana.

 

[barry]

You should explain why.

Uch, right. I'm thinking source termination. I shouldn't post early in the morning.

 

[Easy peasy]

Surely with a matched load - to the cable - there is no reflection ... !?!?

regardless of the source Z.

 

[danadakk]

Source end matters.....symmetrical, one can swap load and generator ends and
result in the same behavior for mismatch.

Impedance matching - Wikipedia

en.wikipedia.org

Source-end conditions[edit]​

At the source end of the transmission line, there may be waves incident both from the source and from the line; a reflection coefficient for each direction may be computed with

{\displaystyle -\Gamma _{ST}=\Gamma _{TS}={Z_{s}-Z_{c} \over Z_{s}+Z_{c}}=\Gamma _{S}\,}

,
where Zs is the source impedance. The source of waves incident from the line are the reflections from the load end. If the source impedance matches the line, reflections from the load end will be absorbed at the source end. If the transmission line is not matched at both ends reflections from the load will be re-reflected at the source and re-re-reflected at the load end ad infinitum, losing energy on each transit of the transmission line. This can cause a resonance condition and strongly frequency-dependent behavior. In a narrow-band system this can be desirable for matching, but is generally undesirable in a wide-band system.


Regards, Dana.

 

[noori.re]

The impedance is not matched. Changing the length of the line will change the observed impedance by the load.